Standing up after prolonged bed rest frequently causes lightheadedness and dizziness. These symptoms can be especially debilitating in patients restricted to bed rest due to various medical conditions. They are also common in astronauts after prolonged space flight. This topic has been the subject of intensive research but the underlying mechanisms remain unclear. A portion of the body's reflex nervous system called the sympathetic nervous system plays an important role in maintaining blood pressure during the changes in body position. When activated, it constricts the blood vessels in the muscles, thus preventing a fall in blood pressure. Prolonged bed rest may lead to altered activation of the sympathetic nervous system. One way of activating the sympathetic nervous system is to apply lower body negative pressure (LBNP), which simulates a change in body position. This study is designed to test the hypothesis that after prolonged bed rest, the LBNP leads to an inadequate activation of sympathetic nervous system, which causes dizziness and lightheadedness.